In the beginning, the telephone companies relayed voice frequency (VF) information over copper cables. As the demand for telephone lines increased and transmission technology improved, a means for digitally transmitting VF information was developed. The digital technology enabled twenty-four VF signals, i.e., the VF signals carried by 24 telephone lines, to be sampled and multiplexed into one DS-1 (1.544 Mb/s rate) digital line. The economical advantage of this is readily apparent. A fairly recent development of this digital advancement is the Digital Loop Carrier (DLC) system.
The DLC system is used in local switching services to connect the subscriber (or customer) to the local central office (CO). VF signals are converted to digital form for transmission over a digital transmission medium. In this digitizing process, supervisory signals are incorporated into the transmitted signal in the form of codes in standardized formats which provide to the receiving end of the medium information about the status of or commands from the other end.
In the overall communication process it is possible to provide two modes of transmission, off-hook and on-hook. Off-hook transmission is defined as transmission during the period the subscriber's equipment is requesting service, has closed the loop, or has seized a telephone line. On-hook transmission (OHT) is defined as transmission during the period before the subscriber has gone off-hook. OHT is essential for providing certain services, such as calling party identification (CPI) and other features. However, RT equipment of the prior art is not capable of providing OHT under all circumstances or in response to all supervisory signaling codes.